1. Field of the Invention
The present invention relates to methods and aqueous treating fluid compositions for treating subterranean formations.
2. Description of the Prior Art
Viscous treating fluids are used in a variety of operations and treatments in oil and gas wells. Such operations and treatments include forming gravel packs in well bores, fracturing producing zones, performing permeability control treatments and the like.
Hydrocarbon producing wells are often stimulated by hydraulic fracturing treatments. In hydraulic fracturing, a viscous fracturing fluid, which also functions as a carrier fluid, is pumped into a subterranean zone to be fractured at a rate and pressure such that one or more fractures are formed in the zone. Proppant particles, e.g., graded sand, for propping the fractures open are suspended in the fracturing fluid and are deposited in the fractures when the fluid's viscosity is reduced below a given point. A viscosity breaker is included in the fracturing fluid whereby the fracturing fluid reverts to a thin fluid that is returned to the surface. The proppant particles deposited in the fractures function to prevent the fractures from closing so that conductive channels are formed through which produced hydrocarbons can readily flow.
Aqueous fracturing fluids are generally viscosified by mixing a hydratable polysaccharide gelling agent with water. For example, guar gum and its derivatives are often used to viscosify aqueous fracturing fluids. Guar gum is a random coil polymer that can be readily crosslinked with various cross-linking agents, e.g., metal ions. Once crosslinked, guar and guar derivatives can form highly viscoelastic gels that approach near zero suspended particle settling rates.
It is desirable to maximize the efficiency of gelling agents in hydraulic fracturing fluids, and this is especially true for guar and guar derivatives. The utility of viscous gels is well known in the petroleum industry. However, it has been demonstrated that gel residue left in fractured formations and in proppant particle packs therein after a fracturing treatment results in reduction in the conductivity of the formation. To minimize the damage caused by the gel residue, efforts have been made to minimize the amount of gelling agent included in fracturing fluids.
To increase the effectiveness of gelling agents in general, ionic groups, for example carboxyl groups, have heretofore been grafted onto gelling agents. Since like charges tend to repel each other, the ionic groups force the flexible coiled polymer to become more linear. Increasing the linearity of a gelling agent results in an enlarged radius of gyration, which in turn results in less gelling agent being required to generate a stable gelled fluid having the desired viscosity. The minimum concentration required to form a stable gelled fluid is sometimes referred to as the critical overlap concentration.
While grafting an ionic group onto a gelling agent increases its radius of gyration, it also renders the gelling agent sensitive to the presence of multivalent metal ions. This sensitivity, in turn, causes the viscosity of the gelling agent in a salt solution to be less than the viscosity in fresh water. Since, aqueous fracturing and other well treating fluids often include chloride salts to inhibit swelling of formation clays or are formed with brine and seawater, the salt sensitivity is undesirable.
More recently, foamed aqueous fracturing fluids containing hydrophobically modified gelling agents, preferably hydrophobically modified guar, have been developed. The viscosifying effect of the hydrophobically modified gelling agent is increased by adding a surfactant such as alpha olefin sulfonate. The surfactant is included in the foamed fluid in an amount in the range of about 0.2% to about 0.75% by weight of the aqueous solution, and the surfactant functions to crosslink the gelling agent molecules and to produce a foamed fracturing gel. An additional conventional cross-linker can be added to the foamed fracturing fluid to increase its viscosity further. The advantage over conventionally crosslinked foamed fracturing gels is that the foam more readily washes away after fracturing. However, the foamed fracturing fluid does not significantly reduce the level of gelling agent utilized therein.
Thus, there are continuing needs for improved methods of treating subterranean zones with viscous aqueous treating fluids whereby the formation of residue is minimized leaving the subterranean formation more conductive to fluid flow.